The present disclosure relates to the optical communication network field, and more particularly to a system and method for allocating bandwidth in remote equipment on a passive optical network (PON).
The conventional access network technologies are copper cable-based e.g. cable modem (CM) or asymmetric digital subscriber loop (ADSL) making it impossible to further improve the data rate. To accommodate future growth in users' multimedia and real-time services, fiber to the home (FTTH) is one of the solutions to this bandwidth bottleneck. As a fiber-based broadband access technology, PON is the most important approach to FTTH. A typical architecture of the PON is illustrated in FIG. 1.
Currently there are three types of PONs, e.g. asynchronous transfer mode (ATM)-based PON (APON) according to ITU-T recommendation G.983, Ethernet-based PON (EPON) according to IEEE standard 802.3ah, and Gigabit PON (GPON) according to ITU-T recommendation G.984. These three PON technologies are now the focus of the broadband optical access network technologies.
As is typical of the EPON architecture, one optical line terminal (OLT) is connected to a number of optical network units (ONUs) or optical network terminals (ONTs) via a shared optical fiber channel. EPON is available in various topologies, such as tree, ring, and bus topologies, of which the tree topology is used most commonly. To the extent no misunderstanding may be caused, the ONU and ONT are collectively called ONU hereafter.
In an EPON, the downstream transmission (OLT→ONU) is made by point-to-multipoint broadcast using a separate wavelength. Traffic going upstream (ONU→OLT) is routed in a multipoint-to-point manner and adopts time division multiple access (TDMA) technology so that each ONU takes a separate communication timeslot to avoid any conflicts. All of the ONUs involved in the transmission form a transmission cycle that generally has a fixed length.
Though there are international standards and products for the three PON technologies, they are defective in that the encapsulation efficiency of IP packets at the data link layer and the quality of service (QoS) are low, and label forwarding is not supported.
A new PON technology is disclosed in Method for Operating and Managing a Passive Optical Network (Chinese Patent No. 1592302), wherein the generalized multi-protocol label switching (GMPLS) technology is introduced into the PON to improve communication efficiency, QoS, and packet forwarding speed. Furthermore, its introduction of the connection-oriented characteristic (label switching path) makes it possible to use the integrated service (IntServ) model in the PON. The role of the IntServ model is to establish a connection in the network for each traffic and to reserve some resources for absolute QoS assurance. This model cannot be used in a core network where traffic is heavy, but is only applicable to a smaller network, such as access network.
One of the existing methods for bandwidth allocation in the PON is adopting the differentiated service (DiffServ) model as the service model in an EPON. The DiffServ model reduces its requirement on network equipment processing capability and thereby improves the network data transmission rate by grouping the traffic into a limited number of priority classes. This model is suitable for a core network. However, the DiffServ can only provide a differentiated service model on a large scale and cannot ensure the QoS of each traffic flow. In the existing EPON technology, traffic is first pooled within ONUs based on the service class and is then placed into different queues for transmission. The OLT assigns a transmit timeslot to each ONU according to the queuing state reported by each ONU. When the transmit timeslot arrives, the queue scheduling mechanism within the ONU will assign a bandwidth to each service class. The scheduling mechanism typically comprises a priority scheduler and a generalized processor sharing-based scheduler.
The defect of this PON bandwidth allocation method is that the bandwidth is scheduled within the ONU based on the service class. Though it is easy to implement, it can only ensure relatively fair access between different service classes and cannot guarantee fair access among different traffic flows. Because the PON access network directly faces the subscribers, who have various complex applications that compete for limited bandwidth, these applications have varied flow characteristics and QoS requirements. In some cases, some application data flows may preemptively seize the bandwidth by increasing the amount of data transmission while other normal data flows are unable to get services. In other cases, some applications have so many data flows that they may cause network congestion, leading to insufficient services for all flows despite the fact that the network can fully meet the service requirements of some of these flows. The method is incapable of flow control and access permission control and may cause a service QoS problem when the load on the network is heavy.
Another PON bandwidth allocation method is adopting ATM as the data carrier protocol in an APON. The ATM technology allows a separate connection to be established between the OLT and ONU for each flow as well as reservation of resources. The technology separates and aggregates the traffic flows via virtual path/virtual channel (VP/VC). The ATM technology provides a good solution to flow control, QoS assurance, security, and improvement of switching speed and billing in an access network. However, its internet protocol (IP) packet encapsulation efficiency may be too low and the protocol layer may be complex. In view of the importance of IP as the future integrated multi-task platform, the ATM technology may no longer be a suitable carrier protocol for an IP-based optical access network. Therefore, the APON technology may be gradually declining.
Still another method for PON bandwidth allocation is the GPON encapsulation mode (GEM)-based GPON that adopts the framing method similar to general framing procedure (GFP). This method has the same characteristics as the ATM method in that both are optimized for time division multiplexed (TDM) services. As with the ATM-based solution, this PON bandwidth allocation method is inefficient in supporting IP and is relatively complicated to implement.